1. Field of the Invention
This invention relates to a semiconductor acceleration sensor device utilizing a resin sealed package and a method for manufacturing the device. More particularly, this invention relates to the device utilizing an MEMS (Micro Electro Mechanical Systems) or an MCP (Multi Chip Package). The MCP is a package on which an MEMS and a semiconductor circuit chip are mounted together.
2. Description of the Related Art
In recent years, with reduced sizes and thicknesses of electronic devices, attention is focused on an MEMS technology, which enables fabrication of sensors or electromechanical parts or the like in a micrometer size.
FIGS. 8A to 8C of attached drawings show cross-sectional views of examples of the constitution of semiconductor acceleration sensor devices utilizing a conventional MEMS package. FIG. 8A illustrates a single-chip type device, FIG. 8B a multi-chip type (a stacked type) device, and FIG. 8C a multi-chip type (a transversely-mounted type) device.
Generally, a hollow ceramic package is used for packaging a semiconductor acceleration sensor device. For example, in the single-chip type device shown in FIG. 8A, an acceleration sensor chip 10 is housed in a hollow ceramic package 20. The acceleration sensor chip 10 is formed based on a semiconductor fabrication process. A weight part 11 is formed through etching in the central area of the bulk portion of a silicon chip. Four beam parts 12 are formed in a cross shape to support the weight part 11 on a surface of the chip. A space 13 is formed between the weight part 11 and surrounding silicon. When the chip is subjected to an acceleration, the beam parts 12 are deformed. Piezoelectric elements formed on the beam parts 12 (not shown in the figure) detect stress, and thereby an acceleration is obtained (calculated). If the beam parts 12 are subject to a stress greater than an allowable value, the beam parts 12 are damaged due to a strain beyond a breaking limit. Therefore, stoppers are provided respectively above the beam parts 12 on the upper surface side of the acceleration sensor chip 10 and below the weight part 11 on the lower surface side of the acceleration sensor chip 10 to restrain displacement within a predetermined range.
The acceleration sensor chip 10 is fixed on a ceramic header 21 of the ceramic package 20. Wires 15, which are metal thin wires, connect electrode pads of the acceleration sensor chip 10 and post sections of the ceramic header 21. A ceramic cap 22 of the ceramic package 20 is fixed on the ceramic header 21, and covers the acceleration sensor chip 10.
The reason the hollow ceramic package 20 is used to package the acceleration sensor device is that the sensitivity and the repeatability of the device are increased when a space 23 within the ceramic cap 22 is kept under vacuum or filled with a gas so that the weight part 11 does not suffer from a drag when accelerated. When the space 13 is filled with an oil or a gel, as long as viscoelasticity characteristics thereof are stable, a drag acts on the weight part 11 can be cancelled out by tuning the characteristics of the oil or the gel.
A semiconductor pressure sensor device disclosed in Japanese Patent Kokai No. 10-170380 is known as an example of such a device as has a space within a semiconductor pressure sensor chip filled with an oil or a gel. In this semiconductor pressure sensor device, a surface of a semiconductor pressure sensor chip is coated with an elastic resin so that the semiconductor pressure sensor chip is protected from contaminants.
In the stacked type device shown in FIG. 8B, a semiconductor circuit chip 16 is fixed on a ceramic header 21. The semiconductor circuit chip 16 performs signal-processing of detection results of stress detected by the acceleration sensor chip 10 to generate a detection signal. Wires 17 connect electrode pads of the semiconductor circuit chip 16 and the post sections of the ceramic header 21. On the semiconductor circuit chip 16 is fixed the acceleration sensor chip 10. Wires 15 connect the electrode pads of the acceleration sensor chip 10 and the electrode pads of the semiconductor circuit chip 16.
In the transversely-mounted type device shown in FIG. 8C, the acceleration sensor chip 10 and the semiconductor circuit chip 16 are fixed on the ceramic header 21. The acceleration sensor chip 10 and the semiconductor circuit chip 16 are covered with the ceramic cap 22, and hermetically sealed.
The semiconductor acceleration sensor devices utilizing the conventional ceramic package 20 have problems (1) to (3) as follows:
(1) The manufacturing cost of the ceramic package 20 shown in FIGS. 8A to 8C becomes high because of the expensive parts used in manufacture. Furthermore, the ceramic header 21 and the ceramic cap 22 need to be hermetically sealed with low-melting-point glass or solder. The sealing requires high-temperature processing at a temperature of 360° C. or higher (400° C. or higher with respect to low-melting-point glass), which results in a change in the properties of the semiconductor circuit chip 16 which is packaged together with the acceleration sensor chip 10 as shown in FIGS. 8B and 8C.
(2) As to an MCP, in the stacked type device shown in FIG. 8B, when the acceleration sensor chip 10 is bonded on the semiconductor circuit chip 16 with organic material, the ceramic header 21 and the ceramic cap 22 also need to be resin sealed (bonded) for convenience of manufacturing processes. This resin sealing causes a problem in moisture resistance in a long term service life test. On the other hand, in the transversely-mounted type device shown in FIG. 8C, the package 20 becomes too large to have satisfactory package density.
(3) To solve the above problems (1) and (2), it is possible to seal the chip(s) with a resin in stead of the ceramic package 20, utilizing the technique of Japanese Patent Kokai No. 10-170380. As shown in FIG. 8C, a device disclosed in Japanese Patent Kokai No. 10-170380 mounts a pressure sensor chip and a semiconductor circuit chip transversely on a substrate. A resin package having an externally-exposed recess (i.e., a chip mounting part) is molded with the whole of the semiconductor circuit chip being resin sealed. The pressure sensor chip is fixed at the recess and kept exposed to the outside to detect external stress. In order to protect the pressure sensor chip from external contaminants, a surface of the pressure sensor chip is coated with an elastic protective resin which does not interfere with pressure transmission.
However, even though the whole of the semiconductor circuit chip 16 shown in FIG. 8C is resin sealed and the surface of the acceleration sensor chip 10 is coated with an elastic resin using the above technique of Japanese Patent Kokai No. 10-170380, there remains a problem of moisture resistance around the acceleration sensor chip 10. Thus, it is still difficult to overcome the problems (1) and (2).